In a continuously shielded or screened high voltage cable, the electric field is uniform along the cable axis, and there is variation in the field only in the radial direction. The spacing of the electric flux lines and the equipotential lines is closer in the region of the conductor than elsewhere, as shown by the following equation: ##EQU1## where E.sub.x =electrical stress at point x, in volts/mil
x=distance from centre of cable in mils PA1 V.sub.o =applied voltage in volts PA1 R=radius of cable over insulation PA1 r=radius of cable conductor
Thus the stress is a function of the geometry of the cable and in practice the insulation thickness is sufficient to maintain the stresses at acceptable levels for the dielectric concerned. The stress is determined such that the cable will operate continuously at normal working voltages and that the discharge level is acceptably low.
When such a cable is terminated or spliced, the screen or shield is removed for a distance determined by the termination or splicing method. The removal of the screen or shield causes a discontinuity of the electrical field at the screen or shield end, resulting in a high electrical stress. For successful use, the high stress caused here must be reduced to about the maximum level within the cable itself in order not to impair the expected life of the system.
In order to relieve this stress, and prevent failure of the cable and termination or splice in service, a number of methods have been developed to provide adequate stress control. Among these methods may be mentioned the use of stress cones (pre-moulded or fabricated type), resistive coatings and nonlinear tapes.
Stress cones extend the shield or screen of the cable by the use of a conducting material such as wire, metal foil or tapes on part of the surface of an insulating cone. The cone may be made from tapes of plastic or paper, epoxy resins, rubbers etc. Stress cones thus increase the diameter of the cable at the discontinuity and hence reduce the stress. However, their application is labor intensive in that they require considerable skill and time during fabrication on the cable.
Pre-moulded stress cones of the slip-over type may also be used. These require interference fits, which in practice means that both cable and cone have to be made to close tolerance for optimum performance. It has also been proposed to make stress cones by the build up of layers of different lengths of heat shrinkable tubing, but such cones are not very practical as this method is very time consuming and introduces the possibility of interlaminar voids.
Resistive coatings on the surface of the insulation from the conductor to the shield will reduce the stress by conducting sufficient current to establish a substantially linear distribution of voltage. The high resistance necessary to achieve this and to avoid dissipating an excessive amount of power is rather critical and must remain a constant value in service in order to be satisfactory. This is very difficult to achieve in practice and such coatings are not now in general use.
Coverings of preformed sleeves, wrapped tapes such as those based on PVC, or lacquer or varnish coatings, having a non-linear electrical characteristic, have also been proposed to provide stress control. These coverings have the disadvantage that, in general, effective stress control is obtained only by careful and skillful application of the covering and that the materials of the covering deteriorate rapidly at elevated temperatures, by thermal degradation, or by differential thermal expansion between the dielectric and the stress control layer.
It has also been proposed to effect stress control by use of heat shrinkable polymeric articles which have dispersed therein materials giving nonlinear electric impedance characteristics. See, for example, British Pat. Nos. 1,470,501; 1,470,502; 1,470,503 and 1,470,504 and corresponding U.S. application to Penneck and Taylor, Ser. No. 453,165 filed Mar. 20, 1974, abandoned in favor of continuation application Ser. No. 671,343 filed Mar. 29, 1976, abandoned in favor of continuation application Ser. No. 904,736 filed May 11, 1978, the disclosures of which are incorporated by reference. The stress control means may take the form of a heat-shrinkable sleeve which is applied to the portion of the stripped cable which extends from the screen for a pre-determined distance over the cable dielectric. It has further been proposed, for example, in German OLS No. 22 63 909, to apply a layer of a semiconductive paste to the inner surface of a heat-shrinkable sleeve. The sleeve is placed over the stripped cable and heated to force the paste into contact with the cable.
It is advantageous that terminations of high voltage cables for indoor use, and very important that those for outdoor use, are protected against moisture and pollutants, if these are present in the surrounding atmosphere. Such protection may take the form of taping or a sleeve of a material in which, if it is at least partly organic in nature, there may be dispersed an anti-tracking filler, for example hydrated alumina. The anti-tracking filler tends to prevent the formation, on the outer surface of the materials, of carbonaceous, electrically conducting deposits.
It is proposed in British Pat. Nos. 1,337,951, 1,284,082 and 1,303,432 that such protection may be in the form of a heat-shrinkable sleeve of a polymeric material having dispersed therein an anti-tracking filler system.
There are in general use two types of power cable, one comprises oil impregnated papers, wherein paper insulation is applied by helically winding many layers of tape over the conductor, followed by extrusion of a seamless lead or aluminum jacket to provide earth continuity and screening, as well as a moisture barrier and mechanical protection; the assembly is then vacuum dried and impregnated. The second has a polymeric dielectric for example polyvinylchloride, polyethylene which may be cross-linked, or ethylene-propylene rubber. These materials are extruded onto the conductor and where required cross-linked subsequently.
In terminating a high voltage cable by means of heat-shrinkable products, it is necessary in the case of an oil-impregnated paper cable to shrink an insulating heat shrinkable sleeve over the papers. A length of stress grading heat-shrinkable tubing is shrunk in place over the end of the shield to extend for some distance over the dielectric or heat-recovered insulating sleeve, sealant is placed on the uncovered portion of the shield and on the uncovered end of the dielectric or insulating heat-recovered sleeve and a protective heat-shrinkable sleeve of anti-tracking material is recovered over the entire stripped portion of the cable. For outdoor use a number of sheds must be provided on the anti-tracking layer or the anti-tracking heat shrinkable sleeve may be provided with integral sheds as described in British Pat. No. 1,530,994 and U.S. Pat. No. 4,045,604. The disclosures of these are incorporated by reference.
Such terminations have been found to be very satisfactory but although they are considerably time-saving over traditional methods as well as having other advantages, they do involve several separate parts, each of which has to be applied in turn to the cable. This process is time consuming and, if a number of components is used, which is often the case, the process is open to operator error. Clearly, the more operations involved, the more errors are likely to arise. From the foregoing, it can be seen that prior art methods for terminating or splicing high voltage cable are not wholly satisfactory.